In recent years, MEMS devices, which can advantageously form microstructures having a higher aspect ratio (processing depth/opening width ratio) as compared with semiconductor devices, have become extensively applied to physical sensors such as pressure sensors and acceleration sensors, mechanisms such as micromirrors and microactuators to which electrostatic force or piezoelectric systems have been applied, and fluid devices such as nozzles for ink jetting.
In the processing and manufacture of these microstructures, semiconductor materials such as silicon are mainly applied. Processing methods include, for example, an anisotropic wet etching method in which a structure is formed by taking advantage of a difference in etching rate of silicon crystal planes, and a dry etching method to which an RIE (reactive ion etching) apparatus of an ICP (induction coupled plasma) system that can realize processing of high-aspect ratio grooves is applied. Processing of MEMS devices can realize the formation of three-dimensional structures and movable structures of silicon, is superior in processing accuracy to machining, and, thus, can be applied to various structures.
Processing by a combination of dry etching with anisotropic wet etching is described in NPL 1. In this literature, in order to remove scallops formed on the side surface of holes formed by dry etching to smooth the side surface, etching is carried out by applying a KOH aqueous solution containing isopropyl alcohol (IPA) mixed thereinto to improve surface roughness on the side surface of holes. Structures to which this method has been applied are applied to dies of molds formed of silicon structures.
In MEMS fluid devices, the arrangement of a plurality of silicon nozzles is advantageous for stirring or mixing of liquids. In order to form microstructures such as silicon nozzle structures, preferably, the end portion of through-holes has a small-hole shape. In conventional structures, a method is generally adopted in which patterning in a nozzle form is carried out on a plane for planar partial etching to form a groove and the structure is planarly applied. In recent years, however, three-dimensional structures are required as nozzles for ink jet printers. PTL 1 proposes a method for simultaneous production of through-holes. PTL 1 describes a method in which, in a dry etching method that can form vertical holes, a multi-layer mask is applied to form openings in a vertical hole form continuously at an end portion of tapered openings, thereby producing a structure including simultaneously produced through-holes. More specifically, in this structure, an inclined plane is formed from the openings, and the end portion of the holes is vertical.
Further, in the MEMS sensors and actuators, individual or a plurality of wirings are extracted from a device substrate so as to provide movable or sensing functions as various sensors and actuators. These MEMS structures are divided into device substrates in which movable portions or sensing portions have mainly been processed, and electrode substrates that function to exchange electric signals. In some cases, these structures further include cap substrates such as lids.
A method in which a wiring is extracted horizontally (transverse direction) from the device substrate, and a method in which a wiring is extracted vertically (longitudinal direction) from the device substrate are considered as a method for extracting a wiring in the MEMS structure. PTL 2 describes a method in which the wiring is extracted vertically, and a through-hole wiring structure is formed at a position away from the structure prepared in the device substrate.